More than a year after Palo Alto voters approved a plan to set aside 10 acres in the Baylands for a possible composting plant, the future of waste management remains very much up in the air, with three companies proposing disparate visions for disposing of local food scraps, yard trimmings and biosolid waste.

The three visions, which the City Council will discuss Monday night, are a response to a request for proposals the city issued last April, five months after voters passed Measure E. The measure "undedicated" 10 acres of Byxbee Park, making the land available for a waste-to-energy plant. Proponents of the measure argued that the city should keep its composting local rather than export it to another community, as has been Palo Alto's practice since after the city closed down its Byxbee Park landfill in 2011. Another major goal of the effort was to retire the obsolete incinerators that currently burn the city's biosolid waste, aka sewage sludge, and that are viewed by every side in the debate as an embarrassing blot on the city's environmentalist record.

Only two of the three proposals, however, would keep composting local and take advantage of the Measure E land. The third one, which came from a company called Synagro, would basically export all three waste streams that the city is hoping to treat locally. Synagro has as its major partner GreenWaste, the city's current waste hauler. Under its proposal, biosolids would be shipped to a facility in Merced County, about 100 miles away from Palo Alto.

The other two proposals would each utilize "wet anaerobic digestion," the type of technology most supporters of Measure E had advocated for and that had been identified as the most promising alternative by a task force that the city had commissioned in 2011 to study the issue. The process involves breaking down organic waste inside an airtight facility to create methane, which could then be used as natural gas or converted to electricity.

One proposal came from Massachusetts-based Harvest Power and would treat food scraps and biosolids in a proposed facility at the Regional Water Quality Control Plant. Yard trimmings would be treated at a 3.8-acre site that is part of the 10 acres undedicated by Measure E. This is the only proposal that would treat all three waste streams -- yard trimmings, food scraps and biosolids -- onsite.

The other proposal, by We Generation, would ship yard trimmings to Newby Island, near Milpitas. We Generation, which has as its main partner a Houston-based company called Cambi, would use a process called "thermal hydrolysis" before wet anaerobic digestion. The process breaks down cell walls within the organic materials, allowing more energy to be released during the anaerobic-digestion process.

Phil Bobel, assistant director of Public Works, said thermal hydrolysis "unlocks the energy that's inside the cells more efficiently."

Harvest Power, on the other hand, proposed combining anaerobic digestion with "thermal drying," a process that takes place after the waste is processed. The company would take the leftovers and turn it into pellets that could be used as soil supplements or fuel sources, Bobel said.

Though these are the only three private offers on the table, the council will have at least one other options to consider during its Monday discussion: none of the above. One alternative outlined in the report involves scrapping RFP responses altogether and pursuing an altogether different approach toward procuring waste-management services.

The first step in the new process would be building a facility near the water plant that focuses only on biosolids. The facility, according to the report, "would allow for the earliest decommissioning of the existing incinerator and would serve as a back-up option for biosolids."

This facility would be the first in a series that the city would pursue. These could ultimately include an anaerobic-digestion plant and a yard-trimmings operation.

The new staff report also suggests that the city might benefit from owning and operating the new waste facility. The city's request for proposals called for a partnership in which the vendor designs, builds, owns and operates the new plant. The idea at the time was to protect the city from risk as it considers new technologies.

Now, given that the main technology proposed through the RFP process -- wet anaerobic digestion -- is tried and true, staff is reconsidering the ownership model. The staff report notes that "the experience of other wastewater facilities suggests that anaerobic digesters are more cost-effective in the long-term when the public entity, like the City, is the owner-operater."